From The Germans in Normandy, by Richard Hargreaves (Pen and Sword, 2006), Kindle Loc. 388-405:
Germany had suffered casualties nearing four million, three out of four of them on the Eastern Front. 1943 had been a punishing year in Russia. Since July alone, Germany had lost more than 1,200,000 men. The losses could not be made good. Even after stripping Italy and especially France, even after sending more than a quarter of a million men from the training schools, even after sending wounded men back to the front, the German Army in Russia still found itself more than 300,000 short.
Short of men in the east, short of men in the west, Germany turned to desperate measures to fill its thinning ranks. Hitler was convinced the rear areas, supply depots, offices and administrations would prove to be a rich source of untapped manhood. He ordered every division, every naval and Luftwaffe unit to comb out men who could be spared duties behind the lines so they could be sent to the front. But combing out the Wehrmacht could not solve all its ills. The losses had simply been too great. In 1943, the German military machine began calling up seventeen and eighteen year olds and relying more and more heavily on foreign ‘volunteers’: Volksdeutsche – ethnic Germans, born outside the Fatherland; Freiwillige – foreign volunteers sympathetic to the Nazi cause – and Hilfswillige or ‘Hiwis’ – auxiliaries, usually Russians or Poles pressed into military service from the occupied territories or recruited from the millions of prisoners of war wasting away in German camps. With the war turning against the Wehrmacht in the east, it was no longer safe to use anti-Bolshevik Russians on the Eastern Front. From the autumn of 1943 onwards, the High Command steadily began swapping German troops behind the Atlantic Wall for these so-called Osttruppen – eastern troops. By the spring of 1944, one in six infantry battalions along the Atlantic Coast was composed of Osttruppen and foreign volunteers – Russians, Poles, Italians, Hungarians, Romanians, Ukrainians among them. On the eastern coast of the Cotentin peninsula, 709th Infantry Division was typical of the second-rate divisions defending the west in 1944. One in five in its ranks was a volunteer from the east. Its commander, Karl Wilhelm von Schlieben, was sceptical. ‘We are asking rather a lot if we expect Russians to fight in France for Germany against Americans.’